President Volodymyr Zelensky of Ukraine was returning from the frontline last week when he joined a pivotal call with President Trump.

Hours earlier, Mr. Zelensky had visited soldiers defending Ukraine’s northeastern Sumy region against Russian ground assaults. Now, stopping in the small city of Romny, he listened on a secure line with other European leaders as Mr. Trump outlined Russia’s proposal to end the war.

Moscow, Mr. Trump told them, was ready to halt hostilities under a deal involving a territorial exchange between Russia and Ukraine, according to a Kyiv-based European diplomat and a top Ukrainian official who were briefed on the call and spoke on the condition of anonymity to discuss sensitive information. Seeing a chance to broker peace, Mr. Trump told those on the line that he would meet one on one with President Vladimir V. Putin of Russia to try to seal an agreement.

Mr. Trump had just brought into play two of Kyiv’s deepest fears: that it would be forced to cede land to Russia as part of a peace deal, and that Mr. Putin would be given a way out of his diplomatic isolation. It was a direct challenge to Ukraine’s core principles that territorial issues be addressed only after a cease-fire and that no deal be concluded without Kyiv.

After Mr. Zelensky left the call, he recorded his usual evening address to Ukrainians, trying to project optimism. “I spoke with President Trump,” he said, speaking from a playground in Romny as the sun set behind him. “Russia now seems to be more inclined toward a cease-fire — the pressure is working.”

But, he cautioned, “the key is to ensure they don’t deceive anyone in the details — neither us, nor the United States.”